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Obese and overweight people require more fuel to transport them and the food they eat, and a literally swelling global population will make this source of greenhouse emissions worse, say UK researchers.

"We are all becoming heavier and it is a global responsibility," Edwards says. "Obesity is a key part of the big picture."

At least 400 million adults worldwide are obese. The World Health Organization (WHO) projects 2.3 billion adults will be overweight and more than 700 million will be obese by 2015.

In their model, Edwards and Roberts pegged 40% of the global population as obese with a body mass index of near 30. Many nations are fast approaching or have surpassed this level, says Edwards.

BMI is a calculation of height to weight, and the normal range is usually considered to be 18 to 25, with more than 25 considered overweight and above 30 obese.

The researchers found that obese people require 1,680 daily calories to sustain normal energy and another 1,280 calories to maintain daily activities, 18% more than someone with a stable BMI.

Because thinner people eat less and are more likely to walk than rely on cars, a slimmer population would lower demand for fuel for transportation and for agriculture, says Edwards.

This would take the pressure off food and energy supplies and reduce greenhouse gases from agriculture and transport, he says.

The researchers now aim to quantify how much a heavier population is contributing to climate change, higher fuel prices and food shortages.

But meanwhile, they call for policies that reduce obesity and the global demand for both fuel and food.

This includes transport policies that promote walking and cycling, they say.

"Decreased car use would reduce greenhouse gas emissions and thus the need for biofuels, and increased physical activity levels would reduce injury risk and air pollution, improving population health," the researchers conclude.